Today's wireless cellular systems have been designed to handle very efficient data transfer between a single user (UE) and a single base station, denoted eNB in 4G systems. These solutions are sufficient at today's carrier frequencies close to 1-2 GHz. In future fifth generation cellular system (5G) a shift towards higher carrier frequencies is a necessity, to be able to utilize the available spectrum, thereby achieving a higher capacity overall.
A consequence of the move to higher carrier frequencies is that the radio propagation is transformed from “diffuse” scattering to beamlike propagation. This leads to sharp diffraction effects and increasingly heavy radio shadowing behind obstacles. This makes it more difficult to obtain uniform coverage from a single 5G base station (denoted eNB in case of LTE evolution and NR in case of the new access in standardization for 5G at higher carrier frequencies). The implication is a need to transmit from multiple non-co-located transmit points, to cover a single cell. Such massive multi-point transmission is generally expected to become a cornerstone in future 5G radio access.
Note also that 5G technology is based on the concept of ultra-lean transmission. This means that control channel data and system information to a very large extent is to be carried by user data, in a piggy backed fashion. For this reason continuous transmission is in some sense needed in order to keep a multi-point transmission path active.
It should be noted that multi-point transmission is also considered for the present 4G LTE system, however the need and also the massiveness of the solutions for 4G are believed to be less than those of future 5G cellular systems. The same is true for the IEEE standards of which WIFI constitute a major part.
In a massive multi-point transmission system, where data is arriving from uplink core network nodes, each involved transmit point needs to be given access to (portions of) this data, for transmission over the wireless interface. In many applications the data is closely related to data simultaneously being transmitted from other transmit points. This is e.g. the case for streaming video, in which case the data for a specific part of the video are needed at the same time (to within a certain pre-determined margin) in the receiving UE.
It should here be noted that the different transmit points may transmit different data, or the same data for diversity gain, or a mix of these alternatives.
One problem that may arise relates to synchronization of the received data in the UE. Data received by the splitter, in the best case will be an ordered set of packets that need to be transmitted to the UE. However, due to non-uniform and very varying delays in the individual flows, the packets received by the UE will in general be out of order. The delay variations that cause the out-of-ordering may be the result of:                Varying queuing delays in the eNBs,        Varying transport network delays, e.g. due to congestion and the technology used for the physical transport, and/or        Varying radio link quality, causing eNB buffer size variation.        
It is stressed that the radio link variations and hence the delay variations are likely to increase in importance for higher 5G carrier frequencies, due to the increasing radio shadowing.
Small timing errors between packets received are automatically handled by the protocols applied which re-orders the packets to re-synchronize and create the correct data sequence. However, if the asynchronism is too large, the protocols will register an error, and request re-transmission of several packets. In some protocol implementations, this may cause re-transmission of out of sequence packets already received, as well as packets still in flight. This will then affect the user experience in a negative way, causing e.g. the streaming video to display erroneously.
Another problem is that the number of users and thereby data flows is expected to increase very significantly in 5G systems, as compared to present 4G systems. This means that the number of controller algorithm instances will also increase significantly. As a consequence the processing requirements need to be small, for the selected controller algorithm.
Yet another problem in 5G systems is that certain types of applications like robotic control over wireless do require a much reduced latency as compared to present 4G systems. This does require very tight flow control at bearer level, to secure the latency specification is met.
A potential problem is also that the ultra-lean transmission paradigm of 5G requires some sort of continuous transmission over all desired transmission paths, to provide signaling of the necessary control information continuously. If this is not the case, the multi-point wireless transmission path that is subject to data starvation would become inactive, due to a potential loss of critical control states, like channel state information or synchronization.
Some prior art solutions concerning delay when providing multi-point transmission are previously known, each of them having different drawbacks, see for example WO2013/167647 A1, GB 2 321 829 A and EP 2 822 334 A1.